Genetic risk is conferred by subtle differences in genes among individuals in a population. Genes differ between individuals most frequently due to single nucleotide polymorphisms (SNP), although other variations are also important. SNP are located on average every 1000 base pairs in the human genome. Accordingly, a typical human gene containing 250,000 base pairs may contain 250 different SNP. Only a minor number of SNPs are located in exons and alter the amino acid sequence of the protein encoded by the gene. Most SNPs may have little or no effect on gene function, while others may alter transcription, splicing, translation, or stability of the mRNA encoded by the gene. Additional genetic polymorphism in the human genome is caused by insertion, deletion, translocation, or inversion of either short or long stretches of DNA. Genetic polymorphisms conferring disease risk may therefore directly alter the amino acid sequence of proteins, may increase the amount of protein produced from the gene, or may decrease the amount of protein produced by the gene.
As genetic polymorphisms conferring risk of particular diseases are uncovered, genetic testing for such risk factors is becoming important for clinical medicine. Examples are apolipoprotein E testing to identify genetic carriers of the apoE4 polymorphism in dementia patients for the differential diagnosis of Alzheimer's disease, and of Factor V Leiden testing for predisposition to deep venous thrombosis. More importantly, in the treatment of cancer, diagnosis of genetic variants in tumor cells is used for the selection of the most appropriate treatment regime for the individual patient. In breast cancer, genetic variation in estrogen receptor expression or heregulin type 2 (Her2) receptor tyrosine kinase expression, determine if anti-estrogenic drugs (tamoxifen) or anti-Her2 antibody (Herceptin) will be incorporated into the treatment plan. In chronic myeloid leukemia (CML) diagnosis of the Philadelphia chromosome genetic translocation fusing the genes encoding the Bcr and Abl receptor tyrosine kinases indicates that Gleevec (STI571), a specific inhibitor of the Bcr-Abl kinase should be used for treatment of the cancer. For CML patients with such a genetic alteration, inhibition of the Bcr-Abl kinase leads to rapid elimination of the tumor cells and remission from leukemia.
Abdominal aortic aneurysm (AAA) is a localized dilatation of the abdominal aorta, that exceeds the normal diameter by more than 50%. The normal diameter of the infrarenal aorta is 2 cm. It is caused by a degenerative process of the aortic wall. The aneurysm is most commonly located infrarenally (90%), other possible locations are suprarenal and pararenal. The aneurysm can extend to include one or both of the iliac arteries. An aortic aneurysm may also occur in the thorax. Atherosclerotic changes of the vessel wall are found in the majority of AAA that are characterized histopathologically by chronic inflammation, destructive remodelling of elastic media and depletion of medial smooth muscle cells resulting in marked weakening of the aortic wall. AAA represents a degenerative process of the arteries leading to their enlargement that is usually asymptomatic with natural history culminating in either a therapeutic intervention or rupture. Rupture of AAA has high morbidity and mortality. The rupture risk increases with the growth rate as well as the size of the aneurysm.
AAA is uncommon in individuals of African, African American, Asian, and Hispanic heritage. The frequency varies strongly between males and females. The peak incidence is among males around 70 years of age, the prevalence among males over 60 years totals 2-6%. The frequency is much higher in smokers than in non-smokers (8:1). Other risk factors include hypertension and male sex. In the US, the incidence of AAA is 2-4% in the adult population. Rupture of the AAA occurs in 1-3% of men aged 65 or more, the mortality being 70-95%.
The exact causes of the degenerative process remain unclear. Known risk factors include genetic factors, hemodynamic influences, atherosclerosis, and various other factors such as infection, trauma, connective tissue disorders, arterities etc. AAAs are commonly divided according to their size and symptomatology. An aneurysm is usually considered to be present if the measured outer aortic diameter is over 3 cm (normal diameter of aorta is around 2 cm). The natural history is of increasing diameter over time, followed eventually by the development of symptoms (usually rupture). If the outer diameter exceeds 5 cm, the aneurysm is considered to be large. For aneurysms under 5 cm, the risk of rupture is low, so that the risks of surgery usually outweigh the risk of rupture. Aneurysms less than 5 cm are therefore usually kept under surveillance until such time as they become large enough to warrant repair, or develop symptoms. The vast majority of aneurysms are asymptomatic. The risk of rupture is high in a symptomatic aneurysm, which is therefore considered an indication for surgery. Possible symptoms include low back pain, flank pain, abdominal pain, groin pain or pulsating abdominal mass. The complications include rupture, peripheral embolisation, acute aortic occlusion, aortocaval or aortoduodenal fistulae. On physical examination, a palpable abdominal mass can be noted. Bruits can be present in case of renal or visceral arterial stenosis.
The main treatment options for asymptomatic AAA are immediate repair and surveillance with a view to eventual repair. Surveillance is indicated in small aneurysms, where the risk of repair exceeds the risk of rupture. As an AAA grows in diameter the risk of rupture increases. Although some controversy exists around the world, most vascular surgeons would not consider repair until the aneurysm reached a diameter of 5 cm. The threshold for repair varies slightly from individual to individual, depending on the balance of risks and benefits when considering repair versus ongoing surveillance. The size of an individual's native aorta may influence this, along with the presence of comorbitities that increase operative risk or decrease life expectancy. Currently, the main modes of repair available for an AAA are open aneurysm repair (OR), and endovascular aneurysm repair (EVAR). Open repair is indicated in young patients as an elective procedure, or in growing or large, symptomatic or ruptured aneurysms. Open repair has been the mainstay of intervention from the 1950's until recently. Endovascular repair first became practical in the 1990's and although it is now an established alternative to open repair, its role is yet to be clearly defined. It is generally indicated in older, high-risk patients or patients unfit for open repair. However, endovascular repair is feasible for only a proportion of AAA's, depending on the morphology of the aneurysm. The main advantage over open repair is that the peri-operative period has less impact on the patient.
Atherosclerosis is the pathology underlying several of mankind's most lethal diseases, such as myocardial infarction and peripheral arterial occlusive disease. Peripheral arterial disease (PAD) is a progressive condition characterized by arterial stenoses and occlusions in the peripheral arteries of the lower limbs that can be symptomatic or asymptomatic. Symptomatic PAD ranges in severity from intermittent claudication to critical limb ischemia and is considered to be a indicator of systemic atherosclerotic disease in the population older than 55 years. Patients with PAD have an increased risk of subsequent myocardial infarction and stroke and are six times more likely to die within 10 years than are patients without PAD. Smoking is the strongest risk factors for PAD, along with Type 2 Diabetes, hypertension and hyperlipidemia (Dormandy, J., et al., Semin. Vasc. Surg., 12:123 (1999); Hooi, J. D., et al., Br. J. Gen. Pract. 49:49 (1999); Hirsch, et al., Circulation 113:e463-654 (2006)). Clinically significant lesions may gradually narrow the peripheral arteries leading to pain on walking usually relieved by rest (claudication), ischemic ulcers, gangrene, and sometimes limb amputation. Medical therapy is generally ineffective but operations bypassing or replacing the lesion with artificial or venous grafts improve blood flow distally, at least until they become restenosed (Haustein, K. O., Int. J. Clin. Pharmacol. Ther., 35:266 (1997)).
The identification of common genetic variants that affect the risk of PAD and AAA may enable the identification of individuals who are at a very high risk because of their inherent increased genetic susceptibility to the disease. In the case of genes related to nicotine metabolism, such increase in risk may, at least in part, be mainly due to the inability of carriers of at-risk variants to quit smoking. Such findings could potentially lead to improved prevention for high risk individuals, and are especially of importance given the high residual risk of these diseases that remains among ex-smokers.